Concerns of global ozone depletion (e.g. WMO, 1999) have led to major international
programs to monitor and explain the observed ozone variations in the stratosphere. In
response to these, as well as other long-term climate concerns, NOAA has established
routine monitoring programs utilizing both ground-based and satellite measurement
techniques (OFCM, 1988).

Selected indicators of stratospheric climate are presented in each Summary from
information contributed by NOAA personnel. A Summary for the Northern Hemisphere is issued
each April, and for the Southern Hemisphere, each December. These Summaries are available
on the World- Wide-Web, at the site

An area of extensive ozone depletion was observed over Antarctica during the Southern Hemisphere
winter/spring of 2001, as has been the case since about the mid-1980s. For 2001, the area covered by
extremely low total ozone values of less than 220 Dobson Units, defined as the Antarctic “ozone hole”,
was the third largest on record for October and November. The ozone hole reached maximum size in September
and remained large through early October, then gradually decreased in size and ended in early December.
October anomalies of greater than 40 percent below the 1979-1986 base period were observed over Antarctica,
with negative anomalies of more than 10 percent also observed over southern South America. Vertical
profiles of ozone amounts measured by balloons over the South Pole at the end of September and early
October 2001 showed essentially total destruction of ozone in the 15-20 km region, similar to observations
during other recent years. The minimum total ozone value of 101 Dobson Units, observed on October 4, 2001 at
the South Pole, was not as low as the record low value of 86 DU observed in 1993. Lower stratosphere
temperatures over the Antarctic region in 2001 were again low. Temperatures below -78 C ( sufficiently low
for polar stratospheric cloud formation) occurred over a large region, thus promoting chemical ozone loss.
At northern mid-latitudes, extensive ozone destruction was observed in the years following the massive
eruption of the Pinatubo volcano (approximately 1992-1996), with smaller losses observed in the past few years.
Uncertainties in the estimates of future ozone depletion include possible coupling to changes in water vapor
and carbon dioxide. However, in the absence of further major eruptions, ozone depletion over much of the
globe is not expected to worsen substantially in the coming decade, because international actions have been
successful in reducing the release of ozone-depleting substances.

I. DATA RESOURCES

The data used for this report are listed below. This combination of complementary data,
from different platforms and sensors, provides a strong capability to monitor global ozone
and temperature.

Method of Observation

Parameter

Ground-Based

Satellite/Instrument

Total Ozone

Dobson

NOAA/ SBUV/2

Nimbus-7/SBUV

Ozone Profiles

Balloon-Ozonesonde

NOAA/ SBUV/2

Nimbus-7/SBUV

Temperature Profiles

Balloon - Radiosonde

NOAA/TOVS

We have used total column ozone data from the NASA Nimbus-7 SBUV instrument from 1979
through February1985; NOAA-9 SBUV/2 from March 1985 to December 1989; NOAA-11 SBUV/2
from January 1989 to December 1993; NOAA-9 SBUV/2 from January 1994 to December 1995;
NOAA-14 SBUV/2 from January 1996 to June 1998; NOAA-11 SBUV/2 from July 1998 to
September 2000; and NOAA-16 SBUV/2 from October 2000. Solar Backscatter Ultra-Violet
(SBUV) instruments can produce data only for daylight-viewing conditions, so SBUV/2 data are not
available at polar latitudes during winter darkness. Increasing loss of NOAA-11 data at sub-polar
latitudes from 1989 to1993 was caused by satellite precession, resulting in SBUV/2 viewing high
latitudes only in darkness. Recent NOAA-11 and NOAA-16 total ozone data have not yet been fully
validated. This impacts trends determined for the recent period.

II. DISCUSSION

Figure 1 displays monthly average anomaly values (percent) of zonal mean total ozone, as a function
of latitude and time, from January 1979 to November 2001. The anomalies are derived relative to
each month's 1979-2001 average. Certain aspects of long-term global ozone changes may be readily
seen. In the polar regions, ozone values have been substantially lower in the 1990s than in the 1980s.
Largest anomalies are shown for the polar regions in each hemisphere in winter-spring months, with
positive anomalies of more than 10 percent in the earlier years changing to negative anomalies of
greater than 10 percent for most recent years. In September 2001, around 75 degrees south latitude,
negative anomalies exceeded 26 percent (more than 52 percent lower than in earlier years), and were
about 20 percent lower than average in October and November 2001. At midlatitudes, the anomalies
also change from largely positive in the early years to negative in the 1990s. Little or no significant
trend is seen over the tropical region, but alternating years of positive and negative anomalies are
seen, as part of a quasi-biennial oscillation. At the end of 2001, positive anomalies were evident in
the tropical region.

A map of monthly average Southern Hemisphere SBUV/2 total ozone for October 2001 is shown
in Figure 2, with lowest ozone values displaced from the pole. "Ozone hole" values (defined as
total ozone values less than 220) are shown over the South Atlantic sector of the Antarctic continent,
with highest ozone over the Antarctic sector near the international dateline. Figure 3 shows the
difference in percent between the monthly mean total ozone for October 2001 and eight (1979-86)
monthly means for October (Nagatani et al., 1988). Negative anomalies in total ozone of up to 40
percent are shown over the South Atlantic sector of the Antarctic continent, and more than 10
percent below average values are also evident over southern South America.

Figure 4 compares for each year since 1979 the ozone hole area average for all days in October
through November. The growth in the ozone hole area from the 1980s to the 1990s is quite
apparent. From a very small area in 1982, October-November average values increased dramatically
to a maximum in 1998 of 16.2 million square kilometers. The October-November 2001 average
ozone hole area value was 15.9 million square kilometers, much larger than the ozone hole area in
2000, and only a little smaller than the two previous years. September data for all years were not
included for this calculation because SBUV/2 data over the South Polar region were not available in
early September for years 1992, 1993, and 1995.

The center of the ozone hole, and associated lowest ozone, is often located close to the South Pole.
Figure 5 shows a time series during 2001 of ozone profiles
over the South Pole, measured using balloon-borne ozone instruments. The appearance of anomalously low
ozone hole values is seen to begin in mid August, with extremely low values evident at the end of September
and in early October. The ozone destruction, especially in the 15 to 20 km region, is dramatic.
Figure 6 illustrates the change in ozone profiles measured
at the South Pole. On 4 October 2001, 101 DU total column ozone amount was observed, the minimum value
for the year 2001. This is compared with the profile on 8 July, with total ozone amount of 271 DU.
The decrease in total ozone between these two dates is 62 percent. The 4 October profile shows nearly
complete destruction of ozone between 14 and 20 km. The figure also shows the region where temperatures
on 4 October were lower than -78 C. In the region of low temperatures and chemical ozone depletion from
enhanced human produced chlorine and bromine, the 4 October profile shows markedly less ozone than the
profile of 8 July. This clearly demonstrates the value of vertical profile information in helping to
understand the ozone depletion phenomenon and the processes responsible for changes in the total
column amounts.

Figure 7 presents a time series at the South Pole of total column ozone, integrated from
balloon-borne ozone measurements. Minimum ozone amounts at the South Pole Station in 2001 are
seen at the end of September and in early October. Total ozone values were not as low during
September as values for this time period for 2000, but values remained very low longer in 2001 than
in 2000. The extremely low total ozone values in early September likely reflected ozone depleted air
which had previously been exposed to sunlight prior to moving over the South Pole.

Antarctic ozone depletion has occurred primarily between the altitudes of 12 and 20 km. This is a
region where polar stratospheric clouds form. Figure 8 shows 12-20 km column ozone integrated
from the balloon-borne ozone measurements at the South Pole. In 2001 the values were generally as
low as in any previous year. Large depletion rates are expected for the next decade or more, after
which declining stratospheric chlorine amounts should result in slow recovery of stratospheric ozone.

Ozone amounts in the lower stratosphere are closely coupled to temperatures through dynamics and
photochemistry. Extremely low stratospheric temperatures (lower than -78 C) over the Antarctic
region are believed to contribute to depletion of ozone, in that low temperatures lead to the presence
of polar stratospheric clouds (PSCs). PSCs enhance the production and lifetime of reactive chlorine,
leading to ozone depletion (WMO, 1999). Daily minimum temperatures over the polar region, 65S
to 90S at 50 hPa (approximately 19 km) are shown in Figure 9. For most of the southern
hemisphere winter 2001, minimum temperatures in the polar region were low, but not near record
low values. However, minimum temperatures in late September, October and November were near
record lows. Minimum temperatures were sufficiently low (lower than -78 C) during May to
November for polar stratospheric clouds to form and allow enhanced ozone depletion, in the presence
of sunlight. Figure 10 shows monthly average temperature anomalies at 50 hPa for three latitude
regions, 25N-25S, 25S-65S, and 65S-90S. For the polar region, temperatures for October and
November were 3 to 4 C lower than the long-term average. Negative temperature anomalies also
predominated over the middle and tropical latitudes of the Southern Hemisphere.

Figure 11 presents time series of the area of the ozone hole, the size of the polar vortex, and the size
of the polar area where lower stratosphere temperatures were below -78C. The 2001 values are
shown along with the average daily values and the maximum and minimum daily values for the most
recent 10 years. During 2001 the area for all three of these indicators was larger than average.
Indeed the ozone hole area and the polar vortex area were among the largest of recent years.

Figure 12 illustrates the direct relationship between the persistence of the ozone hole region and the
persistence of the Antarctic polar vortex. In years when the winter polar vortex persisted later in the
season, the duration into the Spring season of the ozone hole also tended to be extended. For the
year 2001, the persistence of the ozone hole and the persistence of the Southern Hemisphere polar
vortex were among the greatest for the years since 1982. Indeed, 5 out of the most recent 7 years
have had the longest duration of winter vortex and ozone hole.

III. CONCLUDING REMARKS

Very low ozone values were observed over Antarctica again in 2001. Ozone depletion of 10 percent
to more than 40 percent was observed over Antarctica compared to total ozone amounts observed
in the early 1980's. Vertical soundings over the South Pole during late September and early October
2001 again showed complete destruction of ozone at altitudes between 15 and 20 km. Lower
stratosphere temperatures in the winter and spring of 2001 over the Antarctic region were below
average values, and were sufficiently low for ozone production of polar stratospheric clouds within
the polar vortex. The ozone hole area and the PSC area were again among the largest of all previous
years. For the year 2001, the ozone hole and Southern Hemisphere polar vortex persisted into
December, again among the longest duration of years since 1982.

Observations of chloroflourocarbons and of stratospheric hydrogen chloride support the view that
international actions are reducing the use and release of ozone depleting substances (WMO, 1999;
Anderson et al., 2000). However, chemicals already in the atmosphere are expected to continue to
deplete ozone for many decades to come. Further, changing atmospheric conditions that modulate
ozone can complicate the task of detecting the start of ozone layer recovery. The eruption of the
Pinatubo volcano provided an example of such a complication in the 1990s. Based on an analysis of
10 years of South Pole ozone vertical profile measurements, Hofmann et al., (1997) estimated that
recovery in the Antarctic ozone hole may be detected as early as the coming decade. Indicators
include: 1) an end to springtime ozone depletion at 22-24 km, 2) 12-20 km mid-September column
ozone loss rate of less than 3 DU per day, and 3) a 12-20 km ozone column of more than 70 DU on
September 15. However, an intriguing aspect of recent observations of the Antarctic stratosphere
is the apparent trend towards a later breakup of the vortex, as shown in Figure 12. A full explanation
of such meteorological anomalies is not yet available. Continued monitoring and measurements,
including total ozone and its vertical profile, are essential to achieving the understanding needed to
identify ozone recovery.